Carbohydrates are organic compounds which are very important to living things as their energy sources, etc. and which are present most abundantly on the earth. They are composed mainly of monosaccharides. These monosaccharides have typical structures in which 3 to 8 carbon atoms are bonded in a ring and the structures are largely classified into two types.
That is, said structures are classified into aldose (an aldehyde-containing monosaccharide) and ketose (a ketone-containing monosaccharide). Both aldose and ketose are further classified into respective trioses, tetroses, pentoses and hexoses depending upon the carbon atom number of aldose or ketose.
Various reactions using monosaccharides are known. As such a reaction which is used industrially, there is a reaction which comprises isomerizing glucose (grape sugar) (an aldohexose) into fructose (fruit sugar) (a corresponding ketohexose) to produce a high-fructose syrup.
Said high-fructose syrup is a mixture of glucose and fructose, obtained by isomerizing glucose partially. Owing to the partial isomerization of glucose (having low sweetness) into fructose (having high sweetness), the high-fructose syrup has sweetness similar to that of sucrose.
About 70% of the high-fructose syrup is used in cooling drinks and other drinks because fructose contained therein has higher sweetness at lower temperatures, and other portion is used in general foodstuffs as a sweetener. The yearly production of high-fructose syrup in the world is estimated about 8,000,000 kg.
Both glucose and fructose are hexoses similar in structure. Chemical and enzymatic processes have hitherto been proposed for isomerization of glucose into fructose, and it is currently conducted in industry to isomerize glucose into fructose using an isomerization enzyme, namely, glucose isomerase to produce a high-fructose syrup.
That is, starch, for example, corn starch is liquefied; the resulting liquid is subjected to saccharification using glucoamylase to obtain a starch syrup; and passing the starch syrup continuously through an immobilized enzyme obtained by immobilizing, using one of various methods, a glucose isomerase produced by, a microorganism of, for example, Streptomyces genus, to isomerize the glucose contained in said solution into fructose.
The above isomerization reaction is an equilibrium reaction whose equilibrium point is 1 or thereabouts. At the equilibrium point, about 50% of glucose can be isomerized into fructose at a reaction temperature of about 60.degree. C. In order to allow the isomerization to proceed to such a level, however, a considerable length of time is required, the reaction mixture is colored owing to the heating for such a long time, and a high cost is incurred for the purification and condensation steps required for product marketing. Hence, the reaction is terminated when the isomerization has proceeded to a fructose content of about 42%.
As described above, the high-fructose syrup is produced in order to allow glucose of mass production and low cost to have sweetness similar to that of sucrose. However, when the sweetness of sucrose is arbitrarily taken as 100, the above-mentioned high-fructose syrup containing about 42% of fructose (this fructose syrup is hereinafter referred to as 42%-fructose syrup, in some cases) has a sweetness of 95-100 which is slightly insufficient. Therefore, in the above isomerization reaction alone, it is impossible to directly obtain a high-fructose syrup having the same sweetness as sucrose.
Hence, there is currently produced in industry a 55%-fructose syrup having a sweetness of 100-110 by increasing the fructose content in the 42%-fructose syrup to 55%.
In order to produce a 55%-fructose syrup from the 42%-fructose syrup, however, a large apparatus such as a reactor packed with a cation exchange resin is required; moreover, a complicated operation must be conducted, that is, continuous sugar separation is conducted using said reactor to obtain a fructose syrup containing about 95% of fructose and then this fructose is mixed with the 42%-fructose syrup.
Meanwhile, as the isomerization of other compound having an aldose structure into a compound having a ketose structure, there can be mentioned, for example, isomerization of lactose (a dissacharide) into lactulose. In this isomerization, however, unlike the above isomerization of glucose into fructose, no enzyme effective for isomerization of lactose into lactulose has not yet been found; therefore, the isomerization is currently conducted by adding, to lactose, sodium hydroxide of a concentration not exceeding a given level and then heating the mixture at 70.degree. C. or higher to isomerize lactose into lactulose (Japanese Patent Publication No. 2984/1977). This process, however, gives a low isomerization ratio, i.e. a low lactulose yield of 20% or less (this is lower than the fructose yield). In order to obtain a high-lactulose syrup, the process has a problem that the lactulose syrup obtained must be condensed.
An object of the present invention is to provide a process which is free from the above-mentioned problems of prior art and which can isomerize a compound having an aldose structure into a compound having a ketose structure at a high isomerization ratio.
Another object of the present invention is to provide a process which can isomerize a compound having an aldose structure into a compound having a ketose structure without requiring any special apparatus or any complicated operation.
Still another object of the present invention is to provide a process which can isomerize a compound having an aldose structure into a compound having a ketose structure, using an isomerization enzyme or without using any isomerization enzyme.
Still another object of the present invention is to provide a process which can isomerize a compound having an aldose structure into a compound having a ketose structure without employing the condition of heating in alkalinity (this condition is sometimes disadvantageous for isomerization ratio) even when there has been found no enzyme effective for said isomerization.
Still another object of the present invention is to provide an isomerization agent or accelerator effective in the above process.